1. Field of the Invention
The present invention relates to an apparatus for continuously producing a high strength dual-phase steel sheet or strip with a low yield-to-tensile strength ratio (hereinafter called simply "low yield ratio").
In the automobile industry, efforts are constantly being made to reduce the fuel consumption of cars by reducing the weight of car bodies by using high-tensile strength steel sheets. As a result, there has long been a demand in the automobile industry for steel sheets having excellent formability, particularly low yield strength, high elongation and high tensile strength.
The demand for such steels has been particularly high in the U.S.A. where the auto producers are under a legal obligation to reduce car fuel consumption.
To produce a high strength steel sheet having a low yield ratio, there is used as the starting material either a low carbon microalloyed steel or an ordinary steel having C, Si and Mn as the main alloying elements and without additional special alloying elements such as Nb, V, Ti etc. The material is heated and kept in the temperature range above its A.sub.3 point for a short time, namely in the gamma temperature range, or in the temperature range from the A.sub.1 point to the A.sub.3 point, namely in the alpha-gamma temperature range, and cooled at a cooling rate greater than the critical value so as to obtain a dual-phase consisting of polygonal ferrite grains (.alpha.) and finely divided and uniformly dispersed low temperature transformed phase or phases (at least one of bainite and martensite, often with retained austenite). Such dual-phase steels having 50-80 kg/mm.sup.2 tensile strength have been recognized as most suitable for automobile bodies since they can be produced with good consistency by a rather simple production process.
In one conventional apparatus for continuously treating steel strips such as the above type of high strength steel and having a thick gauge, there is used a so-called horizontal treating furnace, as exemplified by the annealing and pickling line (A.P.L.) used for annealing stainless steel strips. This conventional type of apparatus has a disadvantage that the furnace, which is constituted by a heating zone, a soaking zone and a cooling zone, must have a considerable length in order to provide a large-capacity, high-speed production line suitable for mass production. The length of the furnace requires a long shop, which in turn requires a large capital outlay.
Further, because the furnace atmosphere is open to the ambient air, the steel strips being treated therein are very likely to be oxidized. Thus, for removal of the resulting oxide film, it is necessary to provide an acid pickling section composed of an acid pickling tank, a brush scrubber, a neutralizing tank, a hot water rinsing tank and a drying device following the furnace.
2. Summary of the Invention
One object of the present invention is to overcome the disadvantages of the conventional apparatus as described above and to provide a simple and compact apparatus suitable for continuously producing a high strength dual-phase steel sheet having a low yield ratio, which apparatus has a short length and which does not require an acid pickling section.
Another object of the present invention is to provide an apparatus for continuously producing dual-phase steel sheet or strip, which apparatus has a large production capacity and is thus highly desirable for meeting the recent large demand for such steels.